Prior art processors, in particular those directed for use in developing medical x-ray film, typically include developer, fix and wash solutions which are applied to exposed film. The photosensitive material to be developed first passes through a first tank containing a developer, then a second tank containing the fixer solution and finally through a third tank containing the wash solution. These tanks are substantially fluidly isolated from each other. Overflow replenishment is typically provided to each tank so as to replace the chemicals consumed by the film processing. This replenishment process dispenses a small volume of fresh processing solution into the appropriate processing tank, enabling an equal volume of "used" solution to overflow through a weir located typically in an opposite position within the same tank.
It is also known in the prior art to provide a plurality of tanks for each of the development, fix and wash solutions. This type processor is typically referred to as a multistage processor. In such processors the processing solution flows from one tank into the adjacent tank and so forth. For example, there may be provided three fluid processing tanks for holding the developing solution. The film is passed successively through the tanks and the development solution overflows from the first tank to the second tank and from the second tank to the third tank, and finally to drain. Likewise, a plurality of tanks containing the fix and wash solutions may also be provided. The processor may be operated such that the processing fluid flows concurrently, or counter-current with respect to the path of the travel of the film through the processor. Multi-stage concurrent and counter-current processors have been found to be more effective for developing, fixing and washing medical x-ray films. These multi-stage developing processors require very small amounts of fluid, typically 5 to 10 ml. per sheet of film, to be transferred between adjacent processing tanks at regular intervals. Failure to do so would result in improper chemical concentration distributions within the processing tank, and improper processing of the film. Typical means for transferring the solution from one tank to the next tank is accomplished by allowing the fluid simply to pass over a weir from one tank to the adjacent tank. However, several problems occur with such a process. Because of the very low volume measure replenishment rates, the overflow from one tank to the next is quite unpredictable. Such a system would result in fresh replenisher being contained in a single tank until the total volume delivered to that tank becomes quite large. The tank receiving the replenisher would become overly replenished while adjacent tanks become under-replenished. The variability of chemical concentration within each tank would be excessive, making efficient processing control very difficult or impossible to achieve. The situation is further complicated by the fact that termination of flow through a weir is difficult to predict. Flow ceases when the stream exiting the weir detaches from the liquid within the weir and leaves a bolus with an advancing contact angle at the weir exit that is less than the critical advancing contact angle, the contact angle required to advance the bolus through and/or out of the weir. Random vibrations caused by operation of the processor can also affect the initiation and termination of flow between adjacent tanks.
One response to this situation is the utilization of fluid metering pumps to transfer liquid between adjacent processing tanks. This approach is undesirable due to the large number of pumps that would be required, and the need to precisely match the output of each to avoid accumulating or depleting fluids within the tanks.
Applicants have developed a simple and improved apparatus and method for accurately controlling the flow of processing fluids from one tank to the next. The solution provided by the present invention minimizes undesirable chemical transfer by either chemical diffusion or random variations in hydrostatic pressure differences and is also easily maintained.